Photoresists are compositions (often solutions) used in the production of miniaturized electronic devices and circuits such as those commonly referred to as integrated circuits or "chips." Such devices are generally formed of a semiconductor material (of which silicon is typically most common) insulator materials (often silicon dioxide SiO.sub.2) and conductive materials (typically metals). These materials must be formed in very small and yet very precise geometric patterns ("geometries") both as individual devices and as groups of devices or circuits.
A present method for forming the appropriate geometries in such materials is a photographic-related technique in which the pattern to be formed in a material is first designed as a mask. A mask is a set of images fabricated on a transparent substrate (often glass) and having opaque and transparent parts which represent all, or portions, of the predetermined pattern of the semiconductor, metal or insulating material, or any combination of these which is required to form the device or chip.
In a common aspect of this technique, a mask is applied to a material to be patterned after the material has been coated with a light sensitive material generally referred to as a photoresist, or sometimes simply a resist. In order to carry out the required tasks, a photoresist material must be sensitive to light and resistant to particular chemical or physical reactions. Most commonly, projection lithography is used in which light is projected through the mask and onto the surface of a wafer of material. Other mechanisms can be used, for example electron beam irradiation, but for purposes of the present invention, the use of visible or ultraviolet light as the source of electromagnetic energy is an appropriate example, it being understood that other wavelengths of electromagnetic energy, as well as sources such as electron beams, can be used in a manner completely analogous to that described herein.
Typical photoresists are solutions of low molecular weight organic resins in appropriate solvents along with one or more photosensitive compounds. For example, common resists incorporate novolac (phenol-formaldehyde) resins in solvents such as propylene glycol methyl ether acetate or 2-ethoxy ethyl acetate, with photosensitive compounds such as the diazonapthoquinones. When the photoresist solution is applied to a surface, and the solvents evaporated away, a solid photosensitive film remains.
When the solid photoresist is exposed through a mask, photoreactions take place in the exposed portions but not in the unexposed portions. Following chemical treatment, one or the other of the portions is then removed, depending upon the chemical treatment or the pattern desired. The patterned material is then treated in the manner called for by the next particular step in the manufacturing process. Often, such a next step is etching in which the areas coated with the pattern of photoresist are intended to remain, while the areas from which the photoresist has been removed in conjunction with a pattern, are intended to be etched away.
A problem arises, however, in the use of certain etching techniques, particularly reactive ion etching (RIE) and more particularly RIE's containing oxygen. Reactive ion etching's nature is such that it tends to remove the photoresist undesirably along with the desirable removal of the uncoated areas. As a result, the integrity of the pattern, and thus the device and the circuits, is attacked by such etching. Such a result is, of course, generally disadvantageous or even unacceptable.
Accordingly, photoresist compositions which are further resistant to etching in reactive ion etches are useful compositions. Such etch resistant photoresists have been developed and are described for example in U.S. Pat. Nos. 4,968,582 and 5,114,827 to Tranjan, et al, the contents of which are incorporated entirely herein by reference. These patents demonstrate that typical photoresists, when modified with phosphorous-containing compounds are exceptionally resistant to reactive ion etching in oxygen-containing plasmas.
The success of such modified photoresists has, however, raised additional considerations. For example, photoresists typically contain large amounts of the photoactive compound in the resin, and adding an additional etch resistant compound can bring the photoresist solution to a rather high concentration. For example, photoactive and etch resistant modifiers can make up as much as 30 percent or more of a photoresist composition. Such high percentages of solute in the organic solvent can cause problems under certain circumstances from the standpoint of undissolved solids, viscosity, and other problems related to the high concentration.
Therefore, a desirable photoresist would be one in which photoreactivity was present as well as etch resistance, but with the amount of solute minimized as compared to the amount of organic solvent.